PTW designers of the 21st. century may take an insight from this nice picture of a mid-20th. century example of race aerodynamics.

Ignoring the excessive wetteted area and lack of separation at the front end, it is immediately apparant that the rider forms the essential tail area required for 'weathercock stability. Indeed the overall form resembles, to a degree, a much more modern and appropriate form - minus a rider who would need to be fitted into the space between the riders arms.

The snag is that this rider is not attached to the vehicle, except by hanging on to the steering and gravity - the latter a somewhat unreliable force on a PTW due to the vertical acceleration during direction changes. This means that any 'flutter' of the fin area, whether caused by inertial or aerodynamic effects will be efficiently and directly transmitted into the steering.

Whoops! The FIM may have had a point in banning this level of aerodynamics...